linux-kernel - Re: [PATCH] mm/filemap.c: Update comments in filemap

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Date:   Fri, 3 Jun 2022 13:44:44 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Alistair Popple <apopple@...dia.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/filemap.c: Update comments in filemap_fault()

On Fri, Jun 03, 2022 at 05:12:33PM +1000, Alistair Popple wrote:
> +++ b/mm/filemap.c
> @@ -3168,8 +3168,9 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
>  	VM_BUG_ON_FOLIO(!folio_contains(folio, index), folio);
>  
>  	/*
> -	 * We have a locked page in the page cache, now we need to check
> -	 * that it's up-to-date. If not, it is going to be due to an error.
> +	 * We have a locked page in the page cache. Check that it's up-to-date.
> +	 * If not it is because there was no page in the page cache (only likely
> +	 * in the case of MADV_RANDOM) or due to error.

The comment is correct, but the code is buggy!

We should be calling page_cache_ra_order() in do_sync_mmap_readahead(),
even if VM_RAND_READ is set (or !ra_pages, or mmap_miss is high).
At least for one page.  That would allow us to drop the FGP_FOR_MMAP
flag as page_cache_ra_order() will instantiate a folio for us.

Do you want to write that patch or should I do it?